1. Field of the Invention
The present invention relates to a cathode active material, a cathode electrode using the cathode active material and a non-aqueous secondary battery (lithium secondary battery) using the cathode electrode. More particularly, the present invention relates to a non-aqueous secondary battery excellent in cycle characteristics and charge/discharge characteristics as well as to a cathode active material and a cathode electrode giving the same.
2. Description of the Related Art
As a secondary battery for a portable electronic apparatus, a lithium secondary battery which is a non-aqueous secondary battery is put into practical use and is widely prevalent. Further, in recent years, a lithium secondary battery is attracting people's attention not only as a small one for a portable electronic apparatus but also as a large-capacity battery for being mounted on a vehicle or for electric power storage. For this reason, there is an increasing demand for safety, cost performance, long lifetime and the like of the lithium secondary battery.
The lithium secondary battery has a cathode electrode, a anode electrode, an electrolytic solution, a separator and an outer cladding material as principal constituent elements. Also, the above cathode electrode is constituted of a cathode active material, an electroconductive material, a current collector and a binder (binding agent).
Generally, as the cathode active material, a layered transition metal oxide represented by LiCoO2 is used. However, the layered transition metal oxide is liable to provoke oxygen elimination in a fully charged state at a comparatively low temperature around 150° C., and this oxygen elimination can provoke thermal bursting reaction of the battery. Therefore, when a battery having such a cathode active material is used in the portable electronic apparatus, there is a fear that heat generation, fire catching and the like of the battery may occur.
For this reason, in view of safety, lithium manganate (LiMn2O4) having a spinel structure, lithium iron phosphate (LiFePO4) having an olivine structure and the like that are stable in structure and do not release oxygen at an abnormal time are expected.
Also, in view of cost performance, cobalt (Co) has a problem of having a low degree of presence in the earth crust and being expensive.
For this reason, lithium nickelate (LiNiO2), a solid solution thereof (Li(Co1-xNix)O2), lithium manganate (LiMn2O4), lithium iron phosphate (LiFePO4) and the like are expected.
Also, in view of lifetime, layered transition metal oxides have a problem of causing destruction of the structure of the cathode active material by intercalation and deintercalation of Li to and from the cathode active material accompanying charging/discharging. For this reason, because of being stable in structure, lithium manganate (LiMn2O4) having a spinel structure, lithium iron phosphate (LiFePO4) having an olivine structure and the like are expected rather than the layered transition metal oxide.
Therefore, as a cathode active material of a battery considering safety, cost performance, lifetime and the like, the above-described lithium iron phosphate having an olivine structure, for example, is attracting people's attention. However, when lithium iron phosphate having an olivine structure is used as a cathode active material in a battery, there will be a problem of lowering of the charge/discharge characteristics such as insufficient electron conductivity and low average electric potential.
For this reason, for the purpose of improving the charge/discharge characteristics, an active material represented by the general formula AaMb(XY4)cZd (wherein A is an alkali metal; M is a transition metal; XY4 is PO4 or the like; and Z is OH or the like) is proposed (for example, see Japanese Patent Unexamined Publication No. 2005-522009: Patent Document 1).
Also, an active material represented by the general formula LiMP1-xAxO4 (wherein M is a transition metal; A is an element having an oxidation number≦+4; and 0<x<1) in which a P-site is substituted with the element A is proposed (for example, see Japanese Patent Unexamined Publication No. 2008-506243: Patent Document 2).
Also, as a cathode active material for a non-aqueous secondary battery excellent in charge/discharge characteristics at a large electric current, a material represented by the general formula Li1-xAxFe1-y-zMyMezP1-mXmO4-nZn (wherein A is Na or K; M is a metal element other than Fe, Li and Al; X is Si, N or As; Z is F, Cl, Br, I, S or N) is proposed (for example, see Japanese Patent Unexamined Publication No. 2002-198050: Patent Document 3).
Also, as an electrode active material being economical and having a good charging capacity and good rechargeability over multiple cycles at the time of production, a material represented by Aa+xMbP1-xSixO4 (wherein A is Li, Na or K and M is a metal) is proposed (for example, see Japanese Patent Application Publication No. 2005-519451: Patent Document 4).
Further, lithium transition metal phosphorus such as LiFePO4 in which a difference in molar volume between at least two coexisting phases containing a lithium-rich transition metal phosphate phase and a lithium-poor transition metal phosphate phase is about 5.69 is disclosed (for example, see Table 2 of International Publication No. 2008/039170: Patent Document 5).